Consumers are demanding digitization of electronic products and high-performance thereof. Accordingly, the market of electronic products is also demanding to develop thin and lightweight electronic products and high-capacity batteries of high energy density. In addition, in order to cope with future energy and environmental issues, hybrid electric vehicles, electric vehicles, and fuel cell vehicles are being actively developed. As a result, car batteries are required to become larger in capacity.
Secondary batteries including lithium-ion secondary batteries, lithium-ion polymer batteries, and super-capacitors (electric double layer capacitors and the like) of high energy density and large capacity have a relatively high operating temperature range, respectively. In addition, when the second batteries continue to be used at a high-rate charge-discharge state, the temperature rises. Thus, separators that are usually used in these secondary batteries require higher heat-resistance and higher thermal stability than those required in ordinary separators. In addition, the secondary batteries should have excellent cell characteristics such as rapid charge and discharge and high ionic conductivity to respond to low temperature.
The separator is placed between the anode and the cathode of a battery cell to perform an isolation function. The separator maintains an electrolyte solution to thus provide an ionic conduction pathway. The separator has a shutdown function of blocking the pores by melting part of the separator to block electric current if the battery temperature rises up too much.
When the separator is melted as the temperature gets higher, a big hole is created to thus cause a short circuit occur between the anode and the cathode. The temperature is called a short-circuit temperature. Generally, the separator should have a lower shutdown temperature and a higher short-circuit temperature. In the case of a polyethylene separator, the separator is contracted at 150° C. or higher and thus the electrode portion is exposed, to finally cause a short circuit.
Therefore, it is very important for the secondary battery to have both a shutdown function and a neat-resistance performance in order to achieve a high-energy density and large-area secondary battery. In other words, it is required that the separator should have an excellent heat-resistance performance to thus cause small thermal shrinkage and an excellent cycling performance due to a high ionic conductivity.
It is very deficient to use an existing lithium-ion secondary battery using a polyolefin separator and a liquid electrolyte or an existing lithium-ion polymer battery using a polymer electrolyte that has been gel-coated, on a gel polymer electrolyte or a polyolefin separator for a high-energy density and large-capacity secondary battery in terms of the heat-resistance. Therefore, the heat-resistance performance that is required for a high-capacity and large-area secondary battery for automobiles does not meet the safety requirements. In particular, the separator that is obtained by using polyethylene (PE) or polypropylene (PP) is melted at 150° C. or so, to thereby cause poor heat-resistance.
In order to solve this problem, that is, in order to ensure adequate safety for the high-energy density and large-area secondary battery, Japanese laid-open patent publication No. 2005-209570 disclosed a heat-resistant resin bonded polyolefin separator in which a solution of a heat-resistant resin such as aromatic polyamide, polyimide, polyether sulfone, polyether ketone, and polyetherimide having a melting point of, 200° C. or higher is coated on both surfaces of the polyolefin separator and the heat-resistant resin coated polyolefin separator is dipped in a coagulant solution, washed and dried, to thus obtain the heat-resistant resin bonded polyolefin separator. In order to reduce degradation of ionic conductivity, a phase separation agent is contained in the heat-resistant resin solution for granting porosity, and the heat-resistant resin layer is also limited as 0.5-6.0 g/m2.
However, dipping of the heat-resistant resin blocks the pores of the polyolefin separator to accordingly restrict movement of lithium ions. As a result, since the charge-discharge characteristics are degraded, the heat-resistant resin coated polyolefin separator has not met requirements of large-capacity batteries for automobiles, although it has secured the heat-resistance. In addition, although the pores of the polyolefin separator are not blocked due to dipping of the heat-resistant resin, the ionic conductivity for the large-capacity battery is limited since porosity of the widely used polyolefin separator is 40% or so and the pore size is also several tens nanometers (nm) in diameter.
Japanese laid-open patent publication Nos. 2001-222988 and 2006-59717 disclosed a method of manufacturing a heat-resistant electrolyte separator, in which woven or nonwoven fabrics, porous films, etc., of polyaramid and polyimide whose melting point is 150° C. or higher are impregnated with or coated with a polymer gel electrolyte such as polyethylene oxide, polypropylene oxide, polyether, or polyvinylidene, to thus manufacture the heat-resistant electrolyte separator. However, even in this case, the required heat-resistance may be fulfilled, but in terms of ionic conductivity, ionic mobility in a holder or a heat-resistant aromatic polymer layer is still limited similarly to the case of the separator or gel electrolyte of the conventional lithium-ion battery.
Meanwhile, PCT international patent publication No. WO2001/89022 relates to a lithium secondary battery including ultrafine fibrous porous separator and manufacturing method thereof, and disclosed a technology of manufacturing the lithium secondary battery by using a method including the steps of: melting one or more polymers by a porous polymer separator, or dissolving one or more polymers in an organic solvent, to thus obtain a melted polymer or polymer solution; inputting the melted polymer or polymer solution into a barrel of a charge induced electrospinning machine; and charge-induced-electrospinning the melted polymer or polymer solution through nozzles on a substrate, to thereby form the porous polymer separator.
If only a heat-resistant polymer such as cellulose acetate is electrospinned or a heat-resistant polymer mixed with polyvinylidene fluoride (PVdF) that is a swelling polymer is electrospinned, in order to obtain the porous polymer separator, evaporation of a solvent rapidly proceeds during formation of fibers due to nature of the heat-resistant polymer, to thus cause the fibers to be dried very quickly. Accordingly, it is possible to form fibers in a spin nozzle pack of 1 to 10 holes, but fibers are not collected while flying if a multi-hole spin nozzle pack of more than 10 holes for mass production. As a result, since the separators that are obtained by using the multi-hole spin nozzle pack become too bulky, it may be difficult to form the separators and may act as a cause of the trouble of the spin.
In addition, a porous polymer separator proposed in the PCT international patent publication No. WO2001/89022 is obtained by electrospinning a polymer solution that is formed by dissolving one or more polymers in an organic solvent to then be manufactured into 50 μm thick, and then inserting the porous polymer separator between the cathode and anode in order to manufacture a lithium secondary battery to thus achieve integration by lamination. However, the PCT international patent publication No. WO2001/89022 does not concretely teach a content ratio of a heat-resistant polymer and a swelling polymer.
In addition, the Korean laid-open patent publication No. 2008-13208 disclosed a heat-resistant ultrafine fibrous separator and a manufacturing method thereof, and a secondary battery using the same. Here, the heat-resistant ultrafine fibrous separator is manufactured by an electrospinning method, and is made of an ultrafine fiber of a heat-resistant polymer resin having the melting point of 180° C. or higher or having no melting point, or made of an ultrafine fiber of a polymer resin that can be swollen in an electrolyte together with the ultrafine fiber of the heat-resistant polymer resin.
The method of manufacturing the heat-resistant ultrafine fibrous separator includes the steps of: electrospinning a mixed solution that is obtained by mixing a heat-resistant polymer material having the melting point of 180° C. or higher or having no melting point, and a swelling polymer that is swollen in an electrolyte solution, to thus form an ultrafine fiber web consolidated in both a heat-resistant polymer fibrous form and a swelling polymer fibrous form; and performing thermal compression (that is, laminating) of the ultrafine fiber web in the temperature range of 110 to 140° C.
In addition, in the case of the above Korean laid-open patent publication No. 2008-13208, a fibrous content of the swelling polymer material is 95 wt % or less above zero with respect to a polymer composition of the separator, in the heat-resistant ultrafine fibrous separator.